The present invention relates to a generator of encoding or decoding 8-bit bytes, which can in particular be used in a video system.
It is firstly pointed out that in a videography system, such as the ANTIOPE system, the information is coded in the form of bytes constituted by a most significant bit b.sub.8 which is an imparity bit, three most significant bits b.sub.7, b.sub.6, b.sub.5 and four least significant bits b.sub.4 to b.sub.1.
The bytes can be separated into control bytes and data bytes. The control bytes particularly indicate ends and headings of pages, as well as ends and headings of rows. The data bytes, inserted between the control bytes, correspond to characters contained in the rows.
The codes which can be written by means of such bytes are organized according to a table with eight columns according to values of b.sub.5, b.sub.6, b.sub.7 and 16 rows according to values of b.sub.1, b.sub.2, b.sub.3, b.sub.4. In such a table:
columns 0 and 1, which correspond to the bytes, whose 6th and 7th bits are zero, contain the control bytes, PA1 columns 2 and 3 essentially contain the punctuation marks and numbers, PA1 columns 4 and 5 mainly contain capital letters, PA1 columns 6 and 7 mainly contain small letters. PA1 1. it firstly contains a page heading formed by control codes (ETX) FF RS NP.sub.1 NP.sub.2 NP.sub.3, in which ETX is a code finishing the preceding page, FF is a code indicating a new page, RS is a page flag and NP.sub.1 NP.sub.2 NP.sub.3 are codes indicating a page number ranging from 001 to 999; PA1 2. the page heading is followed by a "zero" row: US 00 C.sub.1 C.sub.2 C.sub.3 . . . RC LF, in which US is a row flag, 00 indicates the zero rank, C.sub.1 C.sub.2 C.sub.3 are complementary codes and RC LF finish the row; PA1 3. following the zero row there are information rows firstly consisting of a row heading formed by the codes (RC LF) US NR.sub.1 NR.sub.2, in which RC LF terminate the preceding row, US is a row flag and NR.sub.1, NR.sub.2 constitute the row number ranging from 01 to 24. PA1 c.sub.1, c.sub.2, c.sub.3, c.sub.4, and c.sub.5 are sampled at the output of a pseudo-random sequence generator, reinitialized at the start of each displayable row by a service key K and by the numbers of the page and the row in question, PA1 c.sub.6 and c.sub.7 are at zero, PA1 c.sub.8 is a parity bit. PA1 if the clear byte belongs to columns 0 and 1 of the table of codes (b.sub.6 =b.sub.7 =0), said byte is transmitted as it is and the coding byte is not used, PA1 if the even byte does not belong to columns 0 or 1, it is combined with the coding byte in a logic exclusive OR circuit for forming a locked byte D.sub.j =d.sub.j .sym.C.sub.j, in which the symbol .sym. represents the logic exclusive OR operation. PA1 the group of columns 2 and 3, which essentially contains the punctuation marks and numbers, PA1 the group of columns 4 and 5, which mainly contain capital letters, PA1 the group of columns 6 and 7, which mainly contains the small letters. PA1 the five least significant bits are sampled in a quasi-random sequence obtained by a generator reinitialized for each row heading by the service key K, by the page number and by the row number in question, PA1 the 6th and 7th bits are zero, PA1 the 8th bit is forced to zero. PA1 if the received byte belongs to columns 0 and 1, it is transmitted as it is to the interpretation circuits, PA1 if the received byte does not belong to columns 0 and 1 (because its 6th and/or 7th bits are not zero) it is combined by an exclusive OR circuit with a decoding byte before continuing interpretation.
For example, a running page of information, which is accessible to the user, is in the following form:
There then follows the data bytes d.sub.1, d.sub.2, . . . d.sub.n, which are inserted between the running row heading and either an end of row, start of another row, or an end of page. These bytes d.sub.1, d.sub.2 . . . d.sub.n represent the information constituting the service provided.
French patent application No. FR-A-2 448 824 describes means for only interlocking the information carried in the displayable rows of ordinary pages of the magazine. In other words, interlocking does not affect the control codes appearing in cloumns 0 and 1 of the table of codes referred to hereinbefore and in particular codes ETX, FF, RS, RC, LF and US. It only affects data d.sub.1, d.sub.2, . . . Moreover, the means provided in the aforementioned application respect the imparity of the data bytes.
These results are achieved by the use of an interlocking automaton comprising means for forming a sequence of encoding bytes, whose bits c.sub.1 to c.sub.8 are produced in the following way:
Thus, the coding or encoding sequence obtained is formed by even bytes, designated C.sub.1, C.sub.2, . . . C.sub.n or in generic terms C.sub.j. The plane or clear bytes (i.e. before locking) constiting an information row form a sequence d.sub.1, d.sub.2, . . . d.sub.n or generically d.sub.j, said bytes being uneven or odd. They are locked by the coding bytes in accordance with the following rules. For each new clear byte of the displayable rows, a byte of the coding sequence is sampled:
Due to the parity of the coding bytes, the supplied bytes are uneven, like the clear bytes. Moreover, the exclusive OR operation leads to the substitution of the clear byte by a locked byte belonging to the same group of columns as the clear byte, namely:
Finally, the bytes belonging to columns 0 and 1 are not modified, especially the bytes corresponding to codes ETX, FF, RS, RC, LF and US, which are transmitted as such. Moreover, due to the fact that the 6th and 7th bits of the coding bytes are zero, there is no introduction of new bytes belonging to these columns.
On reception, it is necessary to unlock the information, for which purpose the receiver comprises a decoding byte generator, which supplies the bytes formed in the following way:
The unlocking rules are identical to the locking rules, a new decoding byte being produced for each new received byte belonging to a locked row. Two cases can occur:
After describing the above points, the present invention can be defined. In the aforementioned French Patent Application, generator polynomials combined by a non-linear logic were used for producing the pseudo-random sequence. A system of flip-flops and exclusive OR gates were used. Such a generator indeed supplies coding or decoding bytes adapted to the message to be coded or decoded. However, its performance levels are mediocre, when it is introduced as software into the decoder microprocessor. Thus, in an ANTIOPE decoder equipped with a standard microprocessor, approximately 6 seconds are required for decoding and interpreting a page containing one kilo-byte.